The present disclosure relates generally to a system for, and a method of, controlling illumination of direct part marking (DPM) targets, such as raised, sunken, or etched optical codes, to be electro-optically read by image capture, and, more particularly, to controlling the illumination of a DPM target on a workpiece during operation of a solid-state imaging sensor having a rolling shutter that sequentially exposes an array of pixels to capture an image from the illuminated DPM target.
Direct part marking (DPM) has allowed workpieces to be directly marked, identified and traced to their origin, and its use has been growing in the automotive, aerospace, electronics, medical equipment, tooling, and metalworking industries, among many others. A machine-readable, high-density, two-dimensional, matrix-type, optical code or DPM target is comprised of multiple elements that are directly marked (imprinted, etched, molded, or dot-peened) on a workpiece. Solid-state imaging readers have been used to electro-optically read DPM targets by image capture. The imaging reader includes a solid-state imager (also known as an imaging sensor) with an array of photocells (also known as pixels), which correspond to image elements or pixels over a field of view of the sensor. The sensor is typically a two-dimensional charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device, with global or rolling exposure shutters, and associated circuits for producing and processing electrical signals corresponding to a two-dimensional array of pixel data over the field of view. The imaging reader also includes an illuminating light assembly for illuminating the field of view with illumination light, and an imaging lens assembly for capturing return ambient and/or illumination light scattered and/or reflected from a DPM target in the field of view, and for projecting the return light onto the sensor to initiate capture of an image of the DPM target. A programmed microprocessor or controller analyzes, processes and decodes the DPM target from the captured image.
Although generally satisfactory for its intended purpose, the use of imaging readers for reading DPM targets on workpieces has proven to be challenging. A typical DPM target is relatively small, e.g., less than 3 mm×3 mm. The workpieces themselves often have complicated, i.e., non-planar, curved, reflective surfaces. Contrast between the DPM targets and their workpiece backgrounds, especially from their outer, reflective background surfaces, is still often less than desirable. Unlike machine-readable codes printed in one color (for example, black) on paper of another color (for example, white), DPM targets are typically read not by a difference in intensity of the return light between regions of different color, but by shadow patterns that are cast by the raised or sunken or etched elements. Also, many workpiece backgrounds are dark in color, and absorb incident illumination light.
When using a global shutter sensor where all the pixels are exposed at the same time, it is known to turn the illuminating light assembly on to illuminate the DPM target only during the sensor's exposure time. This results in a very efficient use of the illumination light since the illuminating light assembly is turned off when not needed during non-exposure times. However, when using a lower cost, rolling shutter sensor where the pixels are sequentially exposed at different times, it is known to turn the illuminating light assembly on throughout the time of an entire frame, regardless of the sensor's exposure time, in order to illuminate and capture the entire DPM target image. A typical exposure time is much shorter than the frame time (e.g., for a sensor operating at 30 frames per second, the maximum exposure time could be about 4 ms, while the frame time is 1/30 sec=33.3 ms). This results in a very inefficient use of the illuminating light assembly, especially for sensors having short exposure times and long frames. The additional electrical energy consumed during generation of the illumination light is not only wasteful and energy-inefficient, but also generates undesirable heat, reduces hand motion tolerance, creates undesirably bright illumination light that can annoy operators, and undesirably drains an on-board battery typically provided in handheld, wireless imaging readers, thereby requiring more frequent recharging, more downtime, and shorter working lifetimes. For imaging readers that do not use an on-board battery, the additional electrical energy consumed makes them not readily capable of being powered by a Universal Serial Bus (USB) port that has electrical maximum power limits.
Accordingly, there is a need to more efficiently control DPM target illumination in real-time to reduce average illumination power over a frame, conserve electrical energy, reduce generated excess waste heat, increase hand motion tolerance, reduce the annoyance from bright illumination light, and allow powered operation from a USB port, in the operation of imaging readers having rolling shutter sensors, which are preferred over global shutters, primarily for cost savings, with a minimal impact on reading performance.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and locations of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The system and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.